Conventionally, for internal combustion engines, a lubricating oil is used to smooth the operation of driving apparatus such as automatic speed regulators, shock absorbers, power steering, and gears. Especially, lubricating oils for internal combustion engines (engine oils), besides lubricating various engine parts such as valve gear mechanisms, including mainly piston rings, cylinder liners, bearings of crank shafts and connecting rods, cams, and valve lifters, also act to cool engines, clean and disperse combustion products, and prevent rust and corrosion.
As stated above, various performances are required for lubricating oils for internal combustion engines. Also, recently, under circumstances wherein there are high performances and power outputs in internal combustion engines and wherein they are under severe operating conditions, high performances have been required. Thus to satisfy such required performances, various additives such as a wear-preventive agent, a metal detergent, an ash-free detergent dispersant, an anti-oxidant, etc., are compounded in the lubricating oils for internal combustion engines.
For the fundamental performances of lubricating oils for internal combustion engines it is especially important to make engines act smoothly under various conditions and to prevent wear and seizing. The parts of engines that are lubricated are almost in a fluid-lubricating state. However, the top and bottom dead centers of valve gear systems and pistons tend to be in a boundary lubrication state. The wear-prevention properties under such a boundary lubrication state are typically imparted by adding zinc dithiophosphate (ZnDTP) or zinc dithiocarbamate (ZnDTC).
Since in an internal combustion engine the loss of energy from frictional parts, on which a lubricating oil acts, is high, for measures to decrease friction loss and fuel consumption loss lubricating oils are used in which a friction modifier and various additives are added (for example, see Japanese Patent Publication No. 3-23595). Lubricating oils for automotive internal combustion engines are used under various conditions, such as various oil temperatures, number of revolutions, and loads. Thus to further improve fuel consumption loss they are required to be excellent in friction characteristics under a wide range of used conditions.
Also, for performances that are required in lubricating oils for internal combustion engines, besides those stated above, high heat-resistant properties, a high stability to oxidation, and proper viscosity properties, are exemplified.